Which subject is it the hardest to achieve the doctorate in?

When it comes to applying for a Ph.D., it should be a subject that you are passionate about. As it comes to the difficulty level, they all have their challenges. Theoretical Physics, Biology, and Computer Science have been constantly thrown about as the hardest to obtain, but there are some challengers to this. In order to gain a Ph.D., there is quite a bit of work that goes into it, and then you have to determine an ORIGINAL topic to study in depth. This can be challenging, as research, theories, etc. must be proven in the dissertation, and defended successfully to obtain the degree. Each degree will have its own difficulties and setbacks.

To say that one might be harder than others is hard to answer. While some do not care for math, they are more humanities minded, and vice versa - each doctorate would be extremely challenging to someone who doesn't love that subject.